Many applications require imaging at frame speeds in the range from approximately 2,000 frames/sec to 100,000,000 frames/sec. These include analysis of lightning, sparks resulting from electrical discharge, shock waves, fuel injection and fluid dynamics. Conventional film and video systems are convenient and usually reasonably priced but are only capable of frame rates of up to approximately 100,000 frames/sec (fps).
At ultra high speed (greater than 100,000 fps) a beam splitter or rotating mirror/prism is typically employed together with some shuttering arrangement to record the image at a succession of different physical locations at consecutive points in time. The actual recording can be performed, for example, using conventional film or charge-coupled device (CCD) cameras.
The drawback with film and video techniques is that the systems are usually very clumsy, film systems relying on rotating mirrors or prisms which give limited flexibility in exposure times and frame rate. The cost of developing film can also be very high. Moreover the frame rate must be set for, and cannot be varied during, each “run”.
An electronic technique using a multiple camera approach is described in International patent application publication number WO 95/14951 in which a beam-splitter generates 8 images which are applied to eight different cameras. These cameras may be gated in sequence to capture images at extremely high rates indeed. A wide selection of exposure time and frame rate can usually be made with such an arrangement. A limited number of frames is admittedly provided but this is seldom a drawback in the applications to which such imaging systems are put. Apart from sheer size, the main drawback of such an arrangement is cost. Using 8 optical gates (intensifiers) and 8 CCD cameras, such an imaging system might cost £200,000.